The monitoring of gradient waveforms and more generally of the spatio-temporal magnetic field evolution concurrently with the actual MRI scan has recently been established under laboratory conditions ([1], [2]). It has been shown to be an effective means of correcting image reconstruction from data acquired in the presence of gradient waveform imperfections, eddy currents and field drifts, when combined with adequate image reconstruction algorithms. For the purpose of field monitoring, MR based magnetic field detectors, also called magnetic field probes, are used. When performing the monitoring experiment concurrently with the MRI scan, interference between the MR experiment performed by the MR arrangement and the MR signal generated by the MR field probes can cause various problems. The most important are:                1. Contamination of the signal acquired from the object stemming from the field probe's MR RF signal.        2. Contamination of the signal acquired from the object stemming from the field probe's RF excitation pulse.        3. Contamination of the signal acquired from the field probe by the MR RF signal stemming from the object.        4. Contamination of the signal acquired from the field probe by the power RF signal applied by the MR arrangement to excite the object within.        5. Coupling of MR RF signal between MR based magnetic field probes.        
It is well known that the signals stemming from the observed MR coherences in the object and field probes respectively, can effectively by separated by operating the MR field probes on a nucleus different from the nucleus observed in the actual MR scan ([1-3]). However, the signals used for excitation of the MR object and the MR active substance of the field probe are of substantially higher power than the received MR signals from the object/probe. Therefore the coupling between the transmit RF path of one nucleus and the receiver of the other can impose challenges to the dynamic range of the receiver chain and cause saturation effects in the retrieved signal or even destruction of the receiver. This typically prevents the acquisition of meaningful magnetic field measurements during RF pulses played out by the MR arrangement and also prevents the field probe excitation during object signal acquisition without corrupting the acquired object signal.
In many situations it would be desirable to overturn one or both of the aforementioned constraints. Examples comprise measurements of the magnetic field evolution during spatially selective RF pulses and measurements of the magnetic field evolution over an entire sequence having multiple RF pulses such as (Multi-) Spin-Echo, Steady State Field Precession (SSFP) or Stimulated Echo Acquisition Mode (STEAM). Another example is the excitation of field probes during acquisition phases of the MR scanner without corrupting the acquired object signal, which is particularly desirable when field probes are targeted for rapid and/or interleaved re-excitation ([3], [4], [5]).
In order to prevent the saturation of the receiver chains it is well known that narrow band filters can be used in case different nuclei are applied in the object and the field probes respectively [6]. The filter has to be placed in the signal receive chain such that the first potentially saturating device is protected. Due to the typical amount of coupling (−40 dB to −20 dB) of the MR field probe and transmitters/receivers of the MR scanner, already the first amplification stage (a preamplifier with low noise figure) can suffer from saturation effects and hence the filter has to be placed at its input. The insertion loss of this filter is very critical for the SNR performance of the entire system according to Frii's formula [7]. Typically tradeoffs need to be made between insertion loss in the pass band, stop band depth and compactness of the filter device. Even in the optimal case the remnant insertion loss degrades the SNR retrieved.
Alternatively to filters, trap structures on the coils can be used to reduce the coupling between the two systems [6, 8].
However, in the case the field probes and the MR system operate at the same frequency band, such filters and traps cannot be employed to reduce the net coupling between the two systems.
In some cases the RF power transmission signal strength coupled into receivers or field probes can already cause non-linear effects in passive structures, such as tuning circuits or (variable) capacitors.
An alternative to frequency selective blocking or mutual geometrical or lumped element decoupling is shielding of the MR field probe [9].
A wet-chemically deposited and electroplated thin metal shielding around the magnetic field probe has been proposed to reduce said coupling [9]. A thin (≈22 μm) copper layer was chosen for shielding since the shielding effectiveness at RF frequencies (where the shielding effect is wanted) is only marginally impaired due to the skin effect, while the adverse effect of low (i.e. acoustic) frequency eddy currents running in the metallic surface on the magnetic field measurement is minimized by the low DC conductivity of the thin copper layer. Still, as experienced in [9] the remnant eddy current effects at low frequencies caused the measured temporal field evolution by the probe to be significantly distorted, which was approximated and modeled by a delay. Furthermore this corruption of the measurement was experienced to be anisotropic to the external field direction and spatial modulation. It is to be noted that further reduction of the low frequency eddy currents by reducing the metal layer thickness or the conductivity of the metal impairs the shielding performance at RF frequencies if the layer thickness shrinks to the order of the skin depth at the given RF frequencies. Hence the optimization of low eddy currents at low frequency and high conductivity for shielding at high frequencies is inherently limited by the thin film shielding approach.
Slotting and capacitive coupling of the shield structure is another known approach to improve the performance of MR compatible RF shields [10]. Furthermore metal mesh structures have been employed [11].
While these approaches proved useful to shield an MR coil from the magnet and gradient coil structures, the shielding and eddy current suppression performance for shielding a MR field probe from the coil of the MR scanner was found to be too low.
Therefore, an object of the present invention is to provide an improved arrangement for carrying out dynamic magnetic field measurements in a MR imaging or MR spectroscopy apparatus which allows for improved probe shielding at radio frequencies in the range from about 40 MHz to about 800 MHz as well as for improved suppression of eddy currents.
Yonglai Yang et al. (Nanotechnology, 2007. 18(34): p. 345701-345704; XP 020119505) have proposed using carbon nanostructure based-nanocomposites in order to improve cost efficiency of electromagnetic interference (EMI) shielding in the microwave frequency range of 8.2 to 12.4 GHz (“X-Band”).
WO 2011/070466 A1 discloses various types of RF shields configured as ground plane for an electric circuit board with openings, which is aimed at preventing vibration and heating caused from eddy currents in a magnetic resonance scanner. One type of shields comprises layers of interweaved conductive fibers. However, this kind of shield needs to be provided with gaps between the interweaved fibers in order to provide openings for suppressing time-varying magnetic field gradient-induced vibration.